Fatigue is the most frequently reported unmanaged symptom of cancer patients receiving chemotherapy or radiation therapy. Despite the prevalence of fatigue and its profoundly negative effect upon functional status and quality of life, little research has been published on interventions to prevent or treat fatigue. The purpose of this randomized controlled clinical trial is to test the effectiveness of a nurse-directed walking exercise program to mitigate fatigue and maintain physical functioning during treatment for prostate, breast, or colorectal cancer. Subjects will be stratified by cancer diagnosis and randomized to exercise (EX) or usual care (UC) groups. Subjects in the EX group will be prescribed an individualized home-based walking exercise program that the patients will maintain throughout their cancer treatment. Subjects in the control group will receive the usual care given during cancer treatment. In addition, both groups will receive a booklet, Managing Fatigue. All subjects will be assessed by treadmill and self- report at pretest (baseline before cancer treatment) and at posttest (end of radiotherapy or chemotherapy). In addition, fatigue and other symptoms will be assessed during treatment for both groups. Follow-up symptom assessments and activity levels will be determined at one month, three months, and six months posttreatment. The major outcome variable is fatigue level; additional variables are physical functioning (VO2 max, body composition, muscle strength), emotional distress, difficulty sleeping, and health related quality of life. Groups will be compared by MANCOVA. Multivariate regression procedures will be used to determine the predictors of cancer-related fatigue and of adherence to exercise during cancer treatment. This study is significant as it tests the effectiveness of a low-cost self-care health promotion activity in mitigating fatigue, the most common and distressing symptom of cancer treatment. The biobehavioral outcomes include both subjective self- reported symptoms and objective physiologic changes in functional capacity and performance.